Drum type filtration systems have been conventionally used in the past in applications requiring low flow rates per unit area and large filtration areas. For example, in the filtration of relatively high viscosity liquids such as oil used as coolants or lubricants for steel grinding operations, the separation of metallic or other types of particles from the oil requires relatively low flow rates per unit area of filtration surface. Economy of space therefore makes it desirable to utilize the relatively high filtration surface area per unit of floor area inherent in drum type filters.
The perforate surface of these conventionally used drums may be in the form of wedge wire, or screening or perforated plates with or without a supplementary expendable filter media sock. Additionally, it is conventional to employ a filter aid in powder or fiber form to precoat the filter surface with a low porosity medium for very fine filtration.
The disadvantage of these conventional systems is the need for periodic shut-down of the filtration operation for cleaning or replacement of the filter medium or surface. In order to maintain continuity of the machining operations, which require a continuous supply of filtered coolant or lubricant, it becomes necessary to maintain a large clean tank of filtered liquid as a substitute source during the period of interruption when the filter is shut down. The clean tank necessitates the consumption of additional floor space and the use of additional valving and controls to route the filtered liquid to and from such tank.
Another disadvantage of many existing drum type filter systems is the limitation on the portion of the filter surface which is functionally submerged in the contaminated liquid, because of the need to have a portion of the surface exposed to the air to dry the filter cake so that it can be readily removed.
Accordingly, it is the principal object of the present invention to provide an improved filtration system of the drum type which permits cleaning of the drum by backwashing action without the need for interrupting the filtering process and without the need for maintaining a large capacity clean tank.
These objectives have been accomplished by segmenting the interior of the drum into a plurality of small-volume individual chambers along the interior face of the perforate drum surface. A suction pump draws contaminated liquid through the perforate surface and these peripheral chambers into a main filtered liquid collecting chamber and from there through parts in the stationary central tube upon which the drum is rotatably mounted. Cleaning of the drum is accomplished by a backwashing system which supplies pressurized filtered liquid to a stationary port which is in sliding contact with the open ends of a limited number of the small peripheral chambers, whereby backwashing can proceed through these few chambers simultaneously with continuous filtration through the remaining majority of the chambers. Because filtration proceeds uninterrupted, and because only a small portion of the drum is being backwashed at any given time, only a small volume of clean liquid need be maintained for backwashing. In some cases, it may be possible to eliminate a clean liquid reservoir by simply diverting a small portion of the filtrate from the output of the apparatus directly to the backwashing system.
Backwashing can be continuous, or it can be intermittent under the influence of a pressure sensor which responds to a build-up of excessive pressure drop in the filtrate output line resulting from the excessive accumulation of filter cake to initiate backwashing and indexing of the drum. The disclosed drum type filter system can be used in groups within a tank of dirty liquid, so that, if necessary, one drum can be removed for maintenance while the others continue to operate. Similarly, such tank can also contain supplementary filtering or debris removing devices such as vacuum boxes on the bottom of the tank, drag out chains and the like. A supplementary vacuum box is desirable because it maintains a downward flow in the tank so that filter cake or other debris loosened from the drum surfaces by the backwashing action will not float and re-coat the drum surface.
The novel segmented drum of the present invention also lends itself to use of an optional pre-coating feature by which a filter aid can be applied to the newly cleaned portion of the drum surface after it is indexed away from the backwashing area.
These and other features and advantages of the present invention will become more readily apparent from the following detailed description.